2D defect detection has two primary modes of sample illumination: bright field, which is insensitive to the topography of the wafer, and dark field, which is designed to collect scattered light and is therefore sensitive to the wafer topography and is especially useful for the identification of scratches.
All known dark field techniques illuminate the wafer at a large oblique angle. While suitable for a large range of applications, these techniques fail to enhance a class of defects which is characterized by local low angle inclination of the wafer. This class contains for example cracks and stresses in the wafer which modify the surface.